There are a large number of sound sources in an automobile. From the viewpoint of the demand for quietness from the noise inside and outside a car, various soundproof measures have been taken. In particular, with regard to the components (noise emitting sources) that generate loud sounds, such as engines, transmissions and driving systems, soundproof measures are required in the positions near to the sound sources. Thus, a dedicated soundproof component called a soundproof cover is used.
As conventional soundproof covers, highly rigid covers formed by molding a metal or a resin such as polyamide, polypropylene are used, as planned with putting the principal objective thereof to insulation of the direct noises from the noise emitting sources. Therefore, the covers are so designed that a sound-absorbing material is post-attached to the side of the soundproof cover or a part thereof facing a noise emitting source.
Regarding the evaluation of noises inside and outside an automobile, since the noise level itself is an amount of sense of human, a sound pressure level (dB) obtained by logarithmically compressing an observed sound pressure is used as an evaluation criterion close to an amount of the sound sensed by human. However, when a four (multi)-directional average (combination sound) which is generally employed in a case of evaluating a general soundproofing effect (in point of the increase or decrease in sound pressure level) is calculated, the sound pressure level is affected by the largest sound of all the measured sounds because of the characteristic of the dB sum calculation. Therefore, even though the sound pressure level in only one direction in which a soundproof measure has been taken is reduced, the effect could not be attained as a whole with the result that the sound pressure level as the level of human sense to sound could not be lowered in some cases. Accordingly, it is necessary to thoroughly and uniformly reduce the sound pressure level in every direction.
However, in the above-mentioned soundproof cover constructed by attaching a sound-absorbing material to a rigid cover, the rigid cover may be resonant to vibration transmission (solid-borne sounds) in case where the noise emitting sources vibrate, thereby generating noises by itself to be a new noise emitting source (secondary emission). In general, therefore, the soundproof cover must be fixed to the noise emitting sources via a vibration-insulating material such as rubber bush. Necessarily, therefore there occurs a gap between the peripheral edge of the soundproof cover and the noise emitting source, and inner reverberating sounds (standing waves) leak out from the gap with the result that the sound level reduction could not be attained in some cases.
Accordingly, the present applicant has previously proposed a soundproof cover in which a soft sound-insulating layer formed of a nonwoven fabric coated with a damping resin is provided on the side of the sound-absorbing material opposite to the side thereof to face a noise emitting source, in place of a rigid cover (see Patent Document 1). In this soundproof cover, the noise from a noise emitting source (airborne sounds and solid-borne sounds) runs through the inside of the sound-absorbing material and vibrates the soft sound-insulating layer of the outermost layer for sound emission (secondary emission). As a result, the noise disperses as vibration energy, thereby enhancing the soundproofing performance of the cover.    Patent Document 1 JP-A 2006-98966